PROJECT SUMMARY Robust speech communication requires that listeners learn linguistically-relevant representations for stable language regularities, such as the speech sounds (phonemes) that convey meaning. In an increasingly multilingual society, as many as twenty percent of Americans accomplish this across multiple languages. Yet, second language acquisition is especially challenging among adult language learners, for whom learning typically involves explicit classroom instruction. Troublingly, research documents that instruction routinely results in a `learning plateau' whereby language abilities stagnate or even atrophy despite continued instruction. There is a need to establish effective new approaches to nudge adult language learners off this plateau. This project integrates theoretical and methodological developments in auditory category learning with approaches to classroom-based L2 instruction. Specifically, incidental category learning (in which learners' attention is directed away from to-be-learned categories by an engaging videogame) taps into category learning systems distinct from those engaged in more explicit learning. Moreover, incidental learning of nonspeech sound categories leads to activation of putatively speech-selective cortex associated with speech categorization, suggesting potential representational cross-talk. This guides the central hypothesis of the project: incidental learning of nonspeech perceptual building block categories may provide a `back door' through which to influence adult L2 learners' speech acquisition and to move them off the classroom learning plateau. An intensive 8-week incidental training study with a 3-month retention interval will test the hypothesis (Aim 1). Comparison of incidental nonspeech training with explicit L2 speech training will assess whether this cognitive `back door' may be more effective in promoting L2 speech perception and production than explicit training with L2 speech and will determine the extent to which each interacts with classroom instruction in the L2 (Aim 2). Far transfer and retention of incidental category learning will be established across measures that extend beyond L2 categorization to speech production, word learning, and phonological representation (Aim 3). The results will reveal whether nonspeech, auditory categories sharing common perceptual dimensions with second language categories scaffold L2 acquisition, the degree to which explicit instruction may support or interfere with new auditory categories, whether incidental learning is retained after training, and whether learning gains transfer to support other language-learning tasks. In blending empirical, methodological, and theoretical advances from laboratory studies with explicit classroom learning it will be possible to determine the interplay between incidentally-acquired nonlinguistic perceptual building block categories and an emerging L2. This will advance important theoretical debates about the cross-talk between general auditory representations and speech categories and will provide a novel approach to L2 pedagogy.